Compressors provide a convenient means for compressing gas or steam for many applications. Such compressors may be subject to damaging oscillatory surges when operated under conditions of insufficient flow. If flow through such a compressor decreases below a critical value, herein known as a surge limit, flow momentarily reverses, returns to its original direction and continues oscillating back and forth due to the aerodynamics of such devices. Surge oscillation can produce violent vibration of the apparatus and, in some cases, can actually damage the apparatus.
Insufficiency of flow can occur due to two conditions: (1) insufficient supply; and (2) excessive differential pressure. If the process providing the steam or gas to the compressor is incapable of producing the amount required for smooth compressor operation, the supply limited condition exists. If the compressor feeds a discharge line at a pressure which is too much higher than the inlet pressure to produce the required flow, surge oscillations also result.
One method for controlling surges in a compressor includes the use of a blow-off valve on the outlet of the compressor to encourage adequate flow through the device. Blow-off valves are wasteful of energy since this energy is vented to the atmosphere.
As noted, the flow rate of steam or gas into a compressor is a critical surge parameter. However, the measurement of inlet flow, particularly for steam, is expensive and complicated. The expense is engendered due to the need for precise flow meters as well as by the fact that the measured flow must be corrected for changes in temperature and pressure of the steam.
One industrial application for a compressor includes compressing low-quality steam derived from hot water which is used, for example, as a collant in a primary industrial process. In one such application, hot water, typically below the boiling point, is admitted to a flash tank where the pressure is reduced by compressor suction to below the vapor pressure of water at that temperature. This permits steam to be driven off the water in the flash tank and the temperature of the water to be reduced. This steam is compressed and heated in the compressor and is supplied to a using process which may be, for example, a process heat exchanger, steam heating system, etc.
Slight changes in the inlet hot water flow rate to the flash tank or in the temperature of such water can significantly alter the flow rate of steam generated thereby and fed to the compressor. If the generated steam flow falls to the surge limit, compressor surges will ensue. In addition, if the using process reflects such excessive pressure back to the compressor that the differential pressure across the compressor reduces the flow which can be achieved below the surge limit, surges ensue.